1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuit design, and particularly relates to layout of input/output devices on an integrated circuit die.
2. Description of the Related Art
FIG. 1 provides a representational illustration of a typical integrated circuit (IC) chip (or die) 10 which includes a semiconductor substrate 12, upon which are formed the electronic devices used to implement the chip's functionality. The logic circuitry of the integrated circuit is formed on the interior portion 20 of the semiconductor substrate 12. The logic portion includes a number of functional circuit blocks that can have different sizes and shapes. The larger blocks can include, for example, central processing units such as CPU 21, read-only memories such as ROM 22, clock/timing units such as clock/timing unit 23, random access memories such as RAMs 24, and I/O units such as I/O unit 25 for providing an interface between CPU 21 and peripheral devices. These blocks, commonly known as macroblocks, can be considered as modules for use in various circuit designs, and are represented as standard designs in circuit libraries. The logic portion further includes tens of thousands, hundreds of thousands or even millions or additional small cells 26. Each cell 26 represents a single logic element, such as a gate, or several logic elements interconnected in a standardized manner to perform a specific function. Cells that consist of two or more interconnected gates or logic elements are also available as standard modules in circuit libraries.
Along the periphery of the semiconductor substrate are various input, output or combined input and output (input/output or I/O) devices or cells 16. In a wire-bond IC chip, each such I/O device generally has connected to it at least one metal bonding pad 18 which is used as an electrical connection for an input/output signal. The wire-bond IC die is then mounted within a plastic or ceramic package having multiple pins, and wire connections are made between the die's bonding pads and the package's pins. Finally, the package containing the IC die is mounted onto a printed circuit board in a manner so as to form electrical connections between the pins of the IC and other components on the printed circuit board. In this manner, external signals can be provided to and from the IC die.
Certain of pads 18 are connected to external power (VDD) and ground (VSS). Each such pad is connected to a power I/O cell, which in turn is connected to one of the chip's power and ground rings, respectively. More specifically, power ring 30 and ground ring 31 supply power and ground to the I/O circuitry. Similarly, power ring 32 and ground ring 33 provide power and ground to the internal logic circuitry 20. In order to isolate the internal logic power circuit from the I/O power circuit, certain pad and power I/O cell combinations are connected only to the internal logic power/ground rings, and separate pad and power I/O cell combinations are connected only to the I/O circuit power/ground rings. To further isolate the power/ground supplies for certain sensitive circuits from the power/ground supplies for noisier circuits, cuts are made in the rings (not shown). Each resulting ring segment can then be used to supply a different type of circuit. Also, although only a single I/O power ring is shown, mixed-voltage integrated circuits may utilize a different power ring for each different voltage.
During the physical design stage of conventional IC design, I/O devices 16, internal logic blocks 21-24 and internal logic cells 26 typically are laid out on the semiconductor substrate in such a manner as to make the most efficient use of the available die space, as well as to permit the routing of required connections between the various devices. However, upon completion of physical design and implementation of design rule checking, it was sometimes discovered that the design was not feasible for actual implementation because one or more bonding pads 18 had been placed too close to either neighboring pads or neighboring pad wires (i.e., the wire connecting an I/O device and its pad). In particular, it often was discovered that, due to fabrication limitations, fabrication of the integrated circuit was likely to result in unwanted short circuits. Upon discovering such problems, it was then necessary to go back and revise the physical design, often at significant expense and delay, in order to attempt to eliminate the problem.